The present invention relates to communications, and in particular to an adaptive analog equalizer.
Due to the widespread popularity of the World Wide Web, internet traffic is at an all time high and rapidly increasing. The resulting congestion is taking its toll on users and telephone companies alike. Users are often frustrated by the length of time it takes to download complex graphics and videos. For example, a ten megabyte video clip which is the equivalent of a four minute movie, takes approximately ninety-three minutes using a 14.4 kilobyte modem and forty-six minutes using a 28.8 kilobit modem.
In addition, lengthy data transmissions are tying up telephone company switches that were designed to handle brief telephone calls. Broadband modems, and in particular asymmetric digital subscriber line (ADSL) modems are an emerging technology that promises to dramatically increase the ability to transfer data over conventional telephone lines. Significantly, ADSL modems allow data transfers at rates over two hundred times faster than current modems, and over ninety times faster than ISDN lines.
ADSL was originally conceived of as a technology for delivering interactive multimedia services, such as video on demand over existing telephone networks. However, it is internet access that is currently driving the demand for ADSL. Unlike other high speed data transmission technologies such as ISDN, ADSL requires no massive rewiring or other changes to a telephone company""s local exchange or central office. Notably, ADSL modems use the existing telephone infrastructure, including the so-called xe2x80x9clast milexe2x80x9d of the network, which is the leg from the central office to a subscriber site (e.g., a home or office) that uses a twisted pair of copper lines. Although it is often referred to as the xe2x80x9clast milexe2x80x9d, the leg from the central office to the subscriber site is typically about 12,000-18,000 feet long.
The bandwidth of a conventional copper twisted pair telephone line is approximately 1 MHz. However, conventional analog signals that carry voice over these lines operate in a 4 kHz bandwidth. Advantageously, ADSL takes advantage of the remaining portion of the 1 MHz bandwidth. Specifically, ADSL technology effectively subdivides the 1 MHz bandwidth of the copper twisted pair line into three information channels: i) a high speed downstream channel, ii) a medium speed duplex (upstream/downstream) channel, and iii) a conventional voice channel. Downstream refers to transmissions from the telephone network to the ADSL modem located at a subscriber site, while upstream is the route from the subscriber site to the telephone network. This multichannel approach enables subscribers to access the internet, order a video for viewing and send a facsimile or talk on the telephone all at the same time.
FIG. 1 illustrates a communication system 10 that employs ADSL technology. The system 10 includes a subscriber site 12, which includes a phone 14, a facsimile machine 16 and a personal computer or computer network 18. The subscriber site 12 receives a twisted pair of copper telephone lines 20 that connect the subscriber site with a telephone central office 22. The run length of the telephone line 20 between the subscriber site and the central office 22 is typically 12,000 feet and it could reach 18,000 feet. A POTS splitter 24 located at the subscriber site 12 is connected to the telephone line 20 and couples the telephone line to an ADSL modem 26 and to the phone 14 and facsimile machine 16.
Central office 22 includes a POTS splitter 30 that is operatively connected to an ADSL modem rack 32 and to a public telephone switch 34. As known, the public telephone switch 34 communicates over a public switch telephone network 36. The ADSL modem rack 32 also communicates over the public switch telephone network and is connected via an internet backbone 38 to various devices including a video server 40, a video conferencing server 42 and a World Wide Web server 44.
FIG. 2 is a functional block diagram illustration of the ADSL modem 26 and the POTs splitter 24. The modem 26 includes a hybrid circuit 50 that couples a transmit circuit 52 and a receive circuit 54 to the telephone line.
The transmit circuit 52 includes a digital signal processor (DSP) 56 that provides a digitized transmit signal on a line 58 to a digital-to-analog converter (DAC) 60. The resultant analog signal is input to a low pass filter (LPF) 62 and a filtered transmit signal is provided on a line 64 to the hybrid circuit 50.
The receive circuit 54 receives a signal on a line 66 and includes a high pass filter 68, a programmable gain amplifier 70, a low pass filter 72, an analog-to-digital converter (ADC) 74 and a DSP 76 which together process the signal on the line 66 in a known manner.
The POTs splitter 24 includes a high pass filter 78 and a LPF 80. The LPF 80 has a cut-off frequency set at approximately 4 kHz in order to allow the voice band signal to pass onto the line 28. The HPF 78 filters signals that are transmitted and received by the modem. Therefore, the cut-off frequency of the HPF 78 can be set at no higher than about 30 kHz to ensure that signals from the transmit circuit 52 pass relatively unattenuated through the POTS splitter. In addition, the hybrid 50 is typically used to terminate the HPF 78 in this embodiment.
Attenuation caused by the twisted pair is not constant over frequency spectrum. That is, the telephone line attenuates high frequency components within the received signal spectrum more than lower frequency components. To compensate for signal losses due to the cable/wire, a programmable gain amplifier (PGA) is typically placed in front of the analog-to-digital converter (ADC). The function of the PGA is to amplify the received signal and to increase/maximize the dynamic range of ADC. However, the PGA gain is flat over the frequency band. Therefore, after amplification, the low frequency components will still have a much higher amplitude than the high frequency components. As a result, the dynamic range of ADC is often limited by the low frequency signals. This leads to a situation where the dynamic range of ADC needs to be greater in order to achieve required signal-to-noise ratio (SNR) for system performance.
The amount of signal gain to be provided is further complicated by the fact that signal attenuation increases with the length of the copper wire. Since the distance between the subscriber site and the central office varies considerably (e.g., generally between 12 and 18 kilo-feet), modems at different subscriber sites will see various levels of high frequency signal attenuation. Moreover, signal attenuation is also a function of temperature and copper conditions that are not easily controlled. Hence, modems may experience different degrees of copper loss over time.
Therefore, there is a need for an adaptive equalizer that compensates for the attenuation of the high frequency components, while leaving the lower frequency components relatively unchanged.
An object of the present invention is to provide an adaptive equalizer to compensate for signal attenuation at high frequencies in the receive path of a broadband communications device.
Briefly, according to the present invention, a broadband communications system includes a receive circuit path and a hybrid circuit. The hybrid circuit provides a receive signal to the receive circuit path that comprises an adaptive equalizer circuit, which conditions the received signal and provides a compensated received signal that is processed by the receive path circuit. In a preferred embodiment, the broadband communications system includes a modem.
The adaptive equalizer is an adaptive analog filter that provides different degrees of high frequency boosts to the received signal, while retaining a relatively constant phase shift for each boost setting. The response of the equalizer is controlled by a control circuit (e.g., a digital signal processor) to compensate for the high frequency signal attenuation primarily caused by the signal path. For example, the signal path may include a telephone line between the communications system (e.g., a modem) and the central office. The dynamic response of the equalizer is selected based upon the characteristics of the signal path which the receive signal travels along.
The equalizer may receive single ended or doubled ended signals.
Advantageously, the equalizer conditions the received signal to ensure efficient utilization of the dynamic range of the ADC located in the receive circuit path. The equalizer is suitable for on-chip implementation, resulting in lower cost and power consumption.
These and other objects, features and advantages of the present invention will become apparent in light of the following detailed description of preferred embodiments thereof, as illustrated in the accompanying drawings.